The invention relates to plastic compositions for sheathing a metal body and/or a semiconductor body. The plastics have at least one polymer, in particular a duroplast (thermosetting plastic), and at least one filler. The invention also relates to method for producing fillers and to the fillers themselves. Finally, the invention also relates to plastic composite bodies, which have a base body comprising a metal and/or semiconductor material and have a sheath that has at least one filler and a polymer. Finally, the invention also pertains to a method for detecting the presence of a filler according to the invention in a plastic composite body.
In plastic composite bodies, such as semiconductor components, which have a base body of metal, such as a lead frame, and bodies of semiconductor material, such as a microchip, undesirable failures often occur under actual environmental conditions. In the prior art, this is ascribed above all to a different coefficient of thermal expansion for the material of a sheath of the base body and for the base body itself.
To compensate for differences in the coefficient of thermal expansion of the sheath and of the base body, the prior art often admixed quartz glass with the basic material of the sheath. It is problematic that with the polymers used to fabricate the sheath, adding quartz glass still cannot attain an adequate reduction in the coefficient of thermal expansion of the sheath. If the proportions of quartz glass in the sheath material are too high, problems also occur in molding the sheath.
In the prior art, a different course is therefore fundamentally recommended, namely the development of sheathing materials that adhere better to the base body. Here it is problematic that the polymers developed for this purpose are very expensive and can therefore not be considered for use on a mass production scale.
It is accordingly an object of the invention to provide suitable plastic compositions, fillers, methods for producing suitable fillers, and plastic composite bodies, which overcome the above-mentioned disadvantages of the heretofore-known devices and methods of this general type and which assure reliable operation of the plastic composite bodies, even under changing environmental conditions.
With the foregoing and other objects in view there is provided, in accordance with the invention, a plastic composition for sheathing a metal and/or semiconductor body, comprising:
at least one polymer, in particular a duroplast, and at least one filler;
the filler having spherical SiO2 particles with a graduated diameter distribution defining individual diameter stages, and wherein a standard deviation in a mean diameter of the SiO2 particles of at least one diameter stage is less than 10%.
In accordance with an added feature of the invention, the standard deviation in the mean diameter of the SiO2 particles of at least one diameter stage is less than 5%, and preferably even less than 1.7%.
In other words, it is a first aspect of the invention to provide a filler for the polymer forming the sheath, where spherical SiO2 particles with a graduated diameter distribution are used, and the standard deviation in the mean diameter of the SiO2 particles of at least one diameter stage is less than 10%. Depending on the quality of the production process of the spherical SiO2 particles, standard deviations of the mean diameter of less than 5% and less than 1.7% can also be attained.
In accordance with an additional feature of the invention, a quantity of SiO2 particles of at least one diameter stage and the mean diameters of the individual diameter stages are selected such that a packing density of the SiO2 particles of at least 90% results. A packing density of better than 95% can be achieved by proper selection of these parameters. This results in a plastic composition which has the requisite rheological properties from rolled processing with a low proportion of polymer. The spherical fillers may be provided in bimodal or trimodal particle size distributions. For such a filler, beginning with the most precisely possibly specified primary particle diameter, smaller particles are admixed, whose diameters fit precisely into the interstices thus created in the packing of the primary particles. For a primary particle diameter of d=40 xcexcm, interstices are created that are optimally filled up approximately with secondary beads or balls 9 xcexcm in diameter. The residual voids that then still remain can be filled with a further fraction of correspondingly smaller particles. By using SiO2 particles with a low standard deviation of the mean diameter within a diameter stage, it becomes especially easy to classify the corresponding particles in the smaller diameter stages. It is advantageous that the improvements in the properties of the plastic composition result regardless of which material makes up the primary component of the polymer.
With the above and other objects in view there is also provided, in accordance with an alternative feature of the invention, a plastic composition for sheathing a metal and/or semiconductor body, comprising:
at least one polymer (e.g. duroplast) and at least one filler;
the filler comprising spherical SiO2 particles each having an interior with a center and a surface, and wherein an index of refraction in the interior of at least one of the SiO2 particles increases from the center to the surface.
The same advantages as noted above are obtained in the use of spherical SiO2 particles as a filler in which the index of refraction in the interior of at least one SiO2 particle increases in a direction from its center to its surface. It has been found that when SiO2 particles that have this characteristic are used, plastic compositions that can be processed especially well can also be produced.
The course of the index of refraction or the optical coefficient of refraction in the interior of a spherical SiO2 particle of a sheathing material according to the invention is measured using contrast variation in a suspension of the spherical SiO2 particles. Precisely in spherical SiO2 particles with diameters up to about 100 nm, the courses according to the invention of the optical index of refraction can be readily determined. The method for measuring the index of refraction is based on the fact that the optical contrast of a suspended particle depends on the quantitative difference between the index of refraction of the particle and the index of refraction of the dispersing agent. If the indices of refraction of the particle and of the dispersing agent have the same value, then the contrast equals zero and thus the particle is optically imperceptible. It is possible to vary the optical contrast to suspended particles if the index of refraction of the dispersing agent can be varied continuously by the addition of a miscible second substance. If the dispersing agent is formed by hexane, then benzene can for instance be added as a second substance. If the index of refraction of the particles differs from the index of refraction of the peripheries of the particles, then the contrast of the xe2x80x9cshellsxe2x80x9d of the particles can be reduced to zero by the choice of a suitable index of refraction of the dispersing agent. Then only a nucleus of the particles remains optically observable. In particles with a nonhomogeneous course of the index of refraction, a variation in the optically observable particle size can be attained. By varying the proportions of dispersing agent and second substance, the course of the index of refraction can thus be measured in the interior of the spherical SiO2 particles. For the measurement, a particle size determination is done using a scattered light method, while the index of refraction of the dispersing agent is being varied. From the association between the index of refraction of the dispersing agent and the measured particle diameter, the radius-dependent course of the index of refraction inside the particles can be ascertained.
In accordance with another feature of the invention, the surface of the SiO2 particles has an at least partial modification with trialkoxysilanes. The trialkoxysilanes are advantageously selected from the group consisting of methyltriethoxysilane, ethyltriethoxysilane, hexyltriethoxy-silane, 3-aminopropyltriethoxysilane, 3-glycidoxypropyltrieth-oxysilane, and 3-mercaptopropyltriethoxysilane.
There is also provided a method of producing spherical SiO2 particles, which comprises the following steps:
polymerizing silicic acid in one or more steps with hydrolyzation of tetraalkoxysilanes and forming spherical SiO2 particles;
subjecting a surface of the SiO2 particles at least partially to a modification with trialkoxysilanes, such as methyltriethoxysilane, ethyltriethoxysilane, hexyltri-ethoxysilane, 3-aminopropyltriethoxysilane, 3-glycidoxy-propyltriethoxysilane, or 3-mercaptopropyltriethoxysilane.
In other words, the method for producing the spherical SiO2 particles used according to the invention includes a step of polymerization of silicic acid from the hydrolysis, in particular the alkaline hydrolysis, of tetraethoxysilane (TES). Here, an alkaline hydrolysis of TES is preferably effected in an alcohol or ethanol solution. Aqueously dissolved ammonia as a catalyst is also contemplated, and in a particular course of the method, spherical SiO2 particles are created in accordance with the following reaction equation: 
The silicic acid can be polymerized by adding TES a single time, or by an essentially continuous addition of TES. In the first case beads with diameters in the nanometer range result, while in the second case beads with a larger diameter are created, and a sharply defined size distribution is attained. The step of continuous addition of TES is preferably effected such that the concentration of free TES in a reaction vessel is always below an experimentally determinable nucleation concentration. Advantageously, the step of polymerization of silicic acid is performed at a temperature that is reduced relative to room temperature and may be less than 0xc2x0 C. or less than xe2x88x9220xc2x0 C.
SiO2 particles produced in this kind of sol-gel process have especially strongly activated surfaces, which increase the reactivity with such sheathing materials as polymers considerably. A complete surface coverage with polymers can thus be attained. Because of the reduction in the reaction temperature, the diameter of the resultant silica beads or balls can be increased without increasing their standard deviation in terms of the mean diameter. At a reduced temperature, the tendency to newly form undesired nuclei out of solution in fact no longer exists to the same extent as at room temperature. Thus more TES is available for the further growth of already existing beads. Precisely when the reaction temperature is reduced to xe2x88x9220xc2x0 C., markedly enlarged final particle sizes are thus obtained, and in a good approximation monodisperse bead diameters of 1.7 xcexcm can be attained.
If the addition of the silicate is done continuously over a period of several hours with constant agitation, then the initially formed nuclei of SiO2 grow into beads with diameters in the micrometer range, with a sharply defined size distribution. For optimal growth of the beads, the concentration of free TES in the reaction vessel must always be below a critical nucleation concentration. This can be attained if the inflow of TES is always below or equal to the reaction-dictated rate of degradation of TES. For the controlled addition of defined quantities of TES over an arbitrarily selectable period of time, a metering device with computer-controlled timing is for instance suitable.
For production on a laboratory scale of the SiO2 beads used according to the invention, a mixture of 50 ml of ethanol and 10 ml of ammonia solution is prepared and brought in the cryostat to a temperature of xe2x88x92200xc2x0 C. With constant agitation, a total of 2 ml of TES are added over a period of 10 hours. once the addition is concluded, the solution continues to be stirred for a further two hours. At the end of the procedure, a suspension of spherical particles approximately 1 xcexcm in diameter is available.
Spheres of SiO2 produced in this way can be used as starting particles or seeds for further increasing the bead diameter. To that end, 5 ml of the previously prepared SiO2 suspension are added to a mixture of 50 ml of ethanol and 10 ml of ammonia solution. Analogously to the previous step, a further 2 ml of TES are added to the reaction solution. At the end of this procedure, the monodisperse bead diameter is approximately 1.2 xcexcm. Repetition of the described method steps allows a continual increase in the primary particle diameter. After the fourth repetition of the addition operation, the monodisperse bead diameter is approximately 1.7 xcexcm.
In a concluding step, the NH3 is expelled. To that end, with constant agitation, the solution is kept at a temperature of 50xc2x0 C. for several hours, until pH neutrality is attained. Finally, the suspension is concentrated by centrifuging at moderate rpm to approximately ⅕ the original volume. In this form, the ethanolic particle suspension can be stored or is accessible to surface treatment, for instance by silanization.
Spherical SiO2 particles can thus be produced that have properties that appear especially advantageous for use in plastic composite bodies and their molding compositions. In the methods used in the prior art for producing spherical quartz material in the plasma phase, the superficial silanol groups, in accordance with a discovery that is fundamental to the invention, are maximally hydrolyzed into siloxane groups at the prevailing high temperatures. It has been found that with this quartz material, a surface-covering binding of polymer primers is possible only with difficulty. By means of the invention, it has been found that this property is due to the fact that the siloxane groups are relatively nonpolar and comparatively sluggish to react, which severely impairs the chemical adhesion of adhesion promoters. The surfaces of the particles produced by the method of the invention, in contrast to the prior art, have a high density of reactive silanol groups, which enables a surface- covering adhesion of polymer primers.
In a further feature of the invention, the surface of the SiO2 particles can be modified, for instance with trialkoxysilanes. The following substances are especially attractive for the modification:
methyltriethoxysilane, ethyltriethoxysilane, hexyltriethoxy-silane, 3-aminopropyltriethoxysilane, 3-glycidoxypropyl-triethoxysilane, 3-mercaptopropyltriethoxysilane.
The coating process proceeds analogously to current methods for conventional fillers. However, coating the SiO2 beads described here leads to better results, since the SiO2 involved has more-reactive surfaces.
According to the invention, both SiO2 particles with sharply defined diameter stages and SiO2 particles with nonsharply defined diameter stages can be mixed together to attain the effect according to the invention.
The invention further provides a plastic composite body, in particular an integrated circuit, comprising:
a base body of a metal and/or semiconductor material; and
a sheath of the base body, the sheath being formed of a polymer and at least one filler;
the filler having spherical SiO2 particles with a graduated diameter distribution defining individual diameter stages, and a standard deviation in a mean diameter of the SiO2 particles of at least one diameter stage being less than 10%.
In accordance with the invention, there is also provided a qualitative analysis method for a sheath of a plastic composite body, wherein the sheath is formed of a polymer and has at least one filler with spherical SiO2 particles having a graduated diameter distribution with distinct diameter stages, wherein a standard deviation of the mean diameter of the SiO2 particles of at least one diameter stage is less than a value to be determined. The method comprises the following steps:
separating SiO2 particles out of the sheath of the plastic composite body, such as by melting the SiO2 particles out of the sheath by heat treatment, or by dissolving the SiO2 particles out of the sheath with sulfuric acid or acetone;
preparing a true-to-scale image of the SiO2 particles, such as a micrograph prepared with an scanning electron microscope (SAM);
determining a diameter of the SiO2 particles visible in the image;
assigning the visible SiO2 particles to diameter stages corresponding to a mean diameter thereof; and
calculating a mean value and a standard deviation of the diameter of the SiO2 particles for each diameter stage.
There is also provided, in accordance with the invention and in the context of a plastic composite body with a sheath formed of a polymer and at least one filler with spherical SiO2 particles having a graduated diameter distribution, a method of determining an index of refraction within at least one SiO2 particle. The method comprises the following steps:
separating SiO2 particles out of the sheath of the plastic composite body; and measuring an index of refraction in an interior of at least one of the SiO2 particles and in a direction from a center to a surface thereof.
In other words, the invention also encompasses methods for detecting whether a plastic composition according to the invention has been used to produce a plastic composite body according to the invention. The corresponding detection methodsxe2x80x94here referred to as a qualitative analysis processxe2x80x94include either the steps of determining the mean diameterxe2x80x94and the mean value and standard deviation of the mean diameter, with the aid of the copy of the SiO2 particles dissolved out of the sheath, or measuring the index of refraction in the interior of at least one SiO2 particle. To determine the sphericity of the particles, it is also possible, instead of dissolving the SiO2 particles out of the sheath, to prepare an appropriate fracture surface or polished section face.
The SiO2 particles from the sol-gel process that are used according to the invention can be distinguished as follows, from their appearance in terms of their high monodispersity and sphericity, from conventional spherical quartz material fillings:
A particular characteristic of the SiO2 particles (SiO2 beads) produced by the sol-gel process is the particularly slight variation in particle size about its mean value. This is therefore also said to be a highly monodisperse system.
Another special feature is the high sphericity of these particles. Deviations from the ideal spherical form are expressed in a particle in the fact that the measured primary particle diameter has different values depending on the three-dimensional orientation inside the particle. One measure for the sphericity of a particle is the variation in the different diameter values that occur in one and the same particle. In the case of an ideal sphere, this variation has the value of zero.